The conventional process for preparing polybenzimidazoles is disclosed by C. S. Marvel et al in U.S. Pat. No. 3,174,974. As described in this patent, the process involves a two-stage procedure. In the first stage, the reactants are melt polymerized until a polymer of intermediate molecular weight is obtained. Thereafter, in the second stage of the process, the melt formed polymer is pulverized and subjected to solid state polymerization to produce a higher molecular weight. While this two-step process is the common commercial method for producing polybenzimidazoles, several difficulties are encountered in carrying out the method. The very nature of the two stages of the method, i.e., the melt polymerization step and the solid state powder polymerization step, makes temperature control an important factor. And it is difficult, if not impossible, to control the temperature so as to avoid crosslinking of the polymer. The process for preparing polybenzimidazoles is conducted in an inert atmosphere. During grinding of the prepolymer from the first step, air is absorbed on the surface of the prepolymer. The difficulty in effecting complete removal of this absorbed air before heating poses another disadvantage in the prior art process.
In U.S. Pat. No. 3,784,517, F. L. Hedberg et al disclose a process for preparing polybenzimidazoles that overcomes disadvantages of the above-described process. Thus, Hedberg et al describe a one-step process in which temperature control is unnecessary that comprises carrying out the polymerization in a reaction medium consisting essentially of an aromatic or aliphatic sulfone.
Both of the above-described processes utilize as a monomer fused tetraamines, such as 1,2,4,5-tetraaminobenzene tetrahydrochloride, which exist in various hydrated forms and cannot be recrystallized to a high degree of purity. The free amine which is unstable to both air and light cannot withstand degradation so that the molecular weight of the products is limited.
It is an object of the present invention, therefore, to provide a process for preparing high molecular weight polybenzimidazoles.
Another object of the invention is to provide high purity bis-orthoaminosulfonamide monomers for use in the preparation of polybenzimidazoles.
Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.